1. Field of the Invention
The present invention relates to a sintered annular pellet, and more particularly, to a method for manufacturing a sintered annular nuclear fuel pellet having small tolerances in its outer and inner diameters so that it may be used for a dual-cooled nuclear fuel rod where heat transfer simultaneously occurs at the inner cladding and the outer cladding.
2. Description of the Related Art
A UO2 pellet is most widely used as a nuclear fuel of a commercial nuclear reactor. The UO2 pellet contains a predetermined amount (for example, about 1 to 5 weight %) of U235. While U235 that is being used for the fuel of a nuclear reactor decays due to a neutron, it generates nuclear fission energy. The pellet has a cylindrical shape (for example, diameter: about 9 mm, length: about 10 mm) having hollow dishes at its upper and lower surfaces, and having flat chamfers chamfered at its upper and lower edges.
In a commercial nuclear reactor, a nuclear fuel pellet is used in the form of a rod inserted into a zirconium alloy-cladding having a predetermined length (for example, about 4 m). Such a commercial nuclear fuel rod has limited performance in aspects of temperature and heat flux.
Though the UO2 pellet has various advantages as a nuclear fuel, since the UO2 pellet has a low thermal conductivity compared to metal or nitride nuclear fuel, heat generated by nuclear fission is not rapidly transferred to cooling water and the temperature of the pellet becomes very high. For example, the temperature of the cooling water is in the range of about 320 to 340° C., and the temperature of the pellet is highest in its center and lowest at its surface. The temperature in the center of the pellet in a normally burning nuclear fuel rod is in the range of between 1000 to 1500° C. There is a steep temperature gradient along the radial direction of the pellet.
When a pellet is in a high temperature state, a margin for safety in various design basis accidents is reduced. For example, in a loss of coolant accident, as the temperature of a nuclear fuel directly before the accident is high, the margin for safety becomes small. In addition, when the heat flux of a nuclear fuel rod becomes high, departure of nucleate boiling may occur. Since a bubble layer is formed on the surface of a cladding when departure of nucleate boiling occurs, heat transfer is seriously deteriorated, so that a nuclear fuel rod may be destroyed.
To address the above-described problem, U.S. Pat. No. 3,928,132 (Roko Bujas, titled Annular Fuel Element for High Temperature Reactor, 1975) has suggested an annular nuclear fuel rod, which includes an outer cladding 11, an inner cladding 12 disposed coaxially with the outer cladding 11 and having a smaller diameter than that of the outer cladding 11, and an annular pellet 15 inserted between the outer cladding 11 and the inner cladding 12 as illustrated in FIGS. 1 and 2.
Since the conventional annular nuclear fuel rod 10 allows cooling water to additionally flow along the center, where temperatures are highest in the cylinder-shaped commercial nuclear fuel rod, the average temperature of the nuclear fuel rod dramatically decreases, and also, a heat transfer area per nuclear fuel rod increases dramatically and thus heat flux decreases, so that a thermal margin is expected to improve.
However, since heat generated from an annular pellet of the conventional annular nuclear fuel rod is transferred to cooling water via both the inner cladding and the outer cladding, when more heat is transferred to one of the two sides, heat transferred to the other side is reduced. An amount of the generated heat transferred via one of the two claddings is related to the thermal resistances. Since a much greater amount of heat is distributed and flows to a direction having low thermal resistance, a heat flux of one of the two directions becomes much higher than that of the other.
The thermal resistance of a gap existing between the pellet and the claddings occupies about half of the thermal resistance existing in the annular nuclear fuel rod, and the thermal resistance of the gap is in proportion to the size of the gap.
After manufacturing, gaps de and di between the annular pellet 15 and the claddings 11 and 12 are set to small sizes within a manufacturable range (for example, about 50 to 100 μm) in order to reduce thermal resistance. Recently, as a method for resolving heat flux asymmetry, reducing an inner gap size to 30 μm or less has been suggested.
To obtain the designed gap size, it is very important to minimize the inner and outer diametric tolerances of an annular pellet.
In the conventional nuclear fuel pellet production process, granules or a powder is inserted into a forming mold and then a green compact is manufactured using a double action uni-axial pressing which applies pressure by pressing a vertical forming punch, and then the green compact is sintered. The green compact manufactured using the double action uni-axial pressing process generates sintering deformation such that the diameter of the center is smaller than the diameters of the upper and lower diameters, for example, in the form of a double-headed drum pinched in at the middle or an hourglass during a sintering process. Therefore, a centerless grinding process is performed to allow the pellet to have a constant diameter along the height of the pellet.
The sintering deformation is generated by non-uniform green density distribution which is the most critical problem of the uni-axial pressing. A difference in the green density inside the green compact generates a change in a sintering shrinkage of each part inside the green compact to cause deformation, and in a serious case, may even generate a crack.
FIG. 3 is a schematic view illustrating a green density distribution inside a green compact and pellet shapes depending on a pressing direction.
FIG. 3 illustrates a forming mold 32 and a forming apparatus 30 having an upper punch 31a and a lower punch 31b disposed in the upper and lower portions of the forming mold 32. The non-uniform density distribution generated by forming is due to friction F1 between powder and the forming mold and friction F2 between powder and powder.
Pressure exerted by the forming punches 31a and 31b on the surface of the powder 25 filling the forming mold 32 is lost by friction. Accordingly, actually applied force becomes much smaller than exerted pressure as a distance from a punch surface increases. This region compressed with this low pressure has lower green density than a region where high pressure is exerted. This low density region is densified incompletely or contracts much compared to neighboring other regions during a sintering process.
Green density distribution inside a green compact during a uni-axial pressurizing becomes different depending on a pressing method.
The conventional uni-axial pressing process of granules or putting powder into a forming mold and pressing the powder at the upper and lower sides using punches is schematically illustrated to the right of FIG. 3. The conventional uni-axial forming process can be classified into a single action uni-axial forming and a double action uni-axial forming depending on a pressing direction. Due to friction between powder and powder, and friction between powder and the wall of the forming mold, the pressure exerted by the punches is not uniformly transferred to the inside of the powder, and causes non-uniformity in green density. Green density distribution changes depending on a pressing direction as illustrated in FIG. 3.
In the case of single action pressing, green density reduces toward an upper or lower direction, but in the case of double action pressing, an intermediate portion of a green compact has lowest green density.
The above-described green density non-uniformity causes a different amount of shrinkage after a sintering process. Therefore, the cross-section of a pellet is distorted into a trapezoid, a conical shape, or an hourglass shape as illustrated in FIG. 3.
In the case of a cylindrical pellet, a precise diametric tolerance can be obtained using a centerless grinding process, but in the case of an annular pellet, both the inner and the outer surfaces need to be grinded since a sintering deformation may occur on both the outer surface and the inner surface. The conventional centerless grinding may resolve only outer diametric tolerance, while the grinding process is time consuming and expensive, and the grinding sludge are high-priced enriched uranium. Accordingly, a process for recycling the grinding sludge is also required.
To satisfy the inner diametric tolerance of annular pellet, an inner side needs to be grinded. For grinding the inner side, precise grinding which uses a diamond wheel or a sandblasting process may be used. However, since the inner diameter distribution of an annular pellet is different in every pellet, in the case of grinding using the diamond wheel, unlike centerless grinding, an annular pellet should be taken one by one to perform grinding. Accordingly, productivity is reduced.
A defective product or grinding sludge generated during a nuclear fuel manufacturing process need to be changed into powder through an oxidation process and recycled in general, because enriched uranium is extremely high-priced. However, in case of the sandblasting process, uranium mixes with sand in the grinding sludge, so that difficulties in recycling uranium are to be expected. A problem may be generated in aspects of separating uranium from grinding sludge, and controlling the concentration of impurities.